1. Field of the Invention
The present invention relates to high manganese cast iron containing spheroidal vanadium carbide and method for making thereof, and its object is to provide the high manganese cast iron containing spheroidal vanadium carbide and method for making thereof that had superior mechanical properties such as abrasion-resistance and toughness and nonmagnetic properties by crystallizing spheroidal vanadium carbide in an austenite matrix, and is not needed water toughening heat treating which has been needed when nonmagnetic high manganese steel (high manganese cast steel) is obtained.
2. Description of the Related Art
The high manganese steel (high manganese cast steel) containing manganese which is more than 10 weight % is known as Hadfield steel. The Hadfield steel contains C within the range of 0.9˜1.4 weight %, and Mn 10˜15 weight %, the high manganese iron containing C 1.1˜1.2 weight % and M 12˜13 weight % is manufactured most for an economical reason.
The Hadfield steel can be manufactured by casting, forging or rolling. However, as the Hadfield steel remains molded condition, ferrous carbide precipitates at crystal grain boundary and a part of austenite matrix transforms into martensite. As a result, tensile strength is 400˜500 N/mm2 and elongation is less than 1% and the Hadfield steel becomes embrittled. Consequently, heat treatment(called water toughening) which carries out water quenching around from 1273˜1473 K is necessary (Iron and Steel Institute of Japan. “Heat treatment of steel”. Maruzen Co., Ltd., 1981.p.447-450).
The Hadfield steel manufactured by water-cooling process around at 1273˜1473 K has an austenite matrix, and its toughness, work hardenability and wear-resistance are superior. Further, proof strength is 295 N/mm2, and about 100 N/mm2 larger than 18-8 stainless steel.
Since the Hadfield steel is nonmagnetic, it is used as a structural material of a superconducting device, linear motor track or cryogenic strong magnetic field. Magnetic permeability is less than 1.5 and hardly changes even if the Hadfield steel is machined.
At present, 14 Mn system, 18 Mn system, 25 Mn system, etc. among the high manganese steel (high manganese cast steel) are known, and further, Ni, Cr, Nb, V, N, etc. are added to the high manganese steel according to the purpose or the use.
For example, ASTM A-128 (1969) D etc. to which nickel is added are known. JIS G-5131 (1969) SCMnH11, SCMnH21, ASTMA-128 (1969) C, etc. are known as added Cr. The JIS G-5131 (1969) SCMnH21 etc. is known as added V.
As an example added various alloy elements to the high manganese steel (high manganese cast steel), there is a research report of the solidification structure and solidification process of an alloy which was added C 1.2˜5.0 weight % and V 0˜7.5% to Fe-12 weight % Mn (Akira Sawamoto, et al. “Solidification Structures of High Manganese-Vanadium Cast Steels” Casting. No. 54. Vol 3, 1982.p. 167-172).
On the other hand, in a patent application No. 2001-204291, the applicants provided spheroidal carbide cast iron which is consisted of C 0.6˜4.0 weight %, V 4˜15 weight %, Al 0.05˜1.0 weight %, Mg 0.01˜0.2 weight %, Si 0.2˜4.5 weight %, Cr 13˜30 weight %, Mn 0.2˜3.0 weight %, Ni 4˜15 weight %, remaining iron (Fe) and inevitable impurities and which the covalent binding spheroidal vanadium carbide is crystallized in its structure of cast iron. This spheroidal carbide cast iron had enough properties such as corrosion-resistance, wear-resistance and toughness.
However, in above-mentioned high manganese steel (high manganese cast steel) has following problems. First, the high manganese steel (high manganese cast steel) has caused work hardening on a steel surface by impact load, and has caused wear-resistance. Therefore, its wear-resistance is inferior in circumstances like sliding wear and abrasive wear which do not cause work hardening. Further, when the high manganese steel has been produced by casting, there are much ferrous carbide is precipitated and mechanical properties deteriorated. The heat treatment called water toughening which removes ferrous carbide was required.
Unless heat treatment called water toughening is carried out, the high manganese steel is embrittled and moreover, magnetic permeability is 1.5˜2.5.Therefore, nonmagnetic high manganese steel cannot be obtained.
After a devoted study in order to solve above-mentioned problems, the applicants found that by applying spheroidizing process of the vanadium carbide which has been found out previously by the applicants to the high manganese cast steel, the high manganese cast iron containing spheroidal vanadium carbide crystallized in the austenite matrix is obtained, and this high manganese cast iron is nonmagnetic, and superior mechanical properties such as wear-resistance and toughness without heat treatment called water toughening which is required in manufacturing of conventional high manganese cast iron. As a result, the present invention has been accomplished.